Development and Dosimetry of an "Epithermal" Neutron Beam for Possible Use in Neutron Capture Therapy II. Absorbed Dose Measurements in a Phantom Man

In order to evaluate the usefulness of an epithermal neutron beam (0·5 eV to 10 keV) for the neutron capture therapy procedure, a tissue -equivalent chamber was used to determine separately the depth-dose distributions from γ-rays, fast neutrons, and the 14 N(n, p) 14 C reaction in a phantom man. Results showed that tumours deeper than 2·5 cm could not be treated with a thermal beam (assuming 35 μg of 10 B per gramme of tumour, and a ratio of 10 B in tumour to 10 B in tissue of 3 to 1) because the ratio of tumour dose to maximum normal tissue dose (advantage factor) would have been less than unity. With the epithermal beam on the other hand advantage factors of 1·4 to 1·9 were obtained at all depths greater than 1·5 cm in the phantom head, assuming the same conditions as with the thermal beam. Advantage factors were found to be more sensitive to the absolute 10 B concentration than to the 10 B ratio in tumour to normal tissue. Improved depth-dose curves generated by the epithermal beam can solve the problem of rapid neutron attenuation encountered with thermal neutron beams.